To print a printing substrate—for example a single sheet or a belt-shaped recording material made of the most varied materials, for example paper or thin plastic or metal films—it is known to generate image-dependent charge images on a charge image carrier (for example a photoconductor), which charge images correspond to the images to be printed, comprised of regions to be inked and regions that are not to be inked. The regions of the charge images that are to be inked are made visible as toner images on the charge image carrier via toner particles with a developer station. The toner image that is thereby generated is subsequently transfer-printed onto the printing substrate in a transfer printing zone and is fixed there.
A developer fluid having at least charged toner particles and carrier fluid can thereby be used to ink the charge images. Possible carrier fluids are hydrocarbons or silicone oil, among others.
A method for such an electrophoretic printing in digital printing systems is known from WO 2005/013013 A2 (US 2006/0150836 A1, DE 10 2005 055 156 B3), for example. After the charge images of the images to be printed have been generated on the charge image carrier, these are inked with toner particles by a developer station to form toner images. Here a carrier fluid containing silicone oil with dye particles (toner particles) dispersed in it is thereby used as a developer fluid. The supply of developer fluid to the charge image carrier can take place via a developer roller that supplies the developer fluid via an inking roller. The toner images are subsequently embedded in carrier fluid, accepted by a transfer unit from the charge image carrier, and transferred onto the printing substrate in a transfer printing zone.
In this printing method using developer fluid, the process of electrophoresis is thus used to transfer toner particles in the carrier fluid to the printing substrate, for example via a transfer roller arranged in the transfer unit. The solid, electrically-charged toner particles thereby migrate to the printing substrate via the carrier fluid as a transport medium, wherein the transport can be controlled via an electrical field between the transfer roller and the printing substrate. The layer separates from the carrier fluid after the contact region (nip) between the transfer roller and printing substrate in the depleted region, such that the toner particles are deposited on the printing substrate with high efficiency. In addition to the toner particle charge and the electrical field, a requirement for this is the provision of a sufficiently thick carrier fluid layer through which the toner particles can migrate.
In the transfer of the toner image it is a goal to deliver the toner particles onto the printing substrate together with optimally little carrier fluid. The transfer process should thereby function given the most different types of printing substrates. The printing substrates can differ in a number of properties. For example, coated papers that have a smooth surface are used as printing substrates since a coating color is applied on their surface. The coating color contains pigments that can have different size and shape. With the aid of the size and shape of the pigments, the system of the capillaries or the system of the pores (designated only as capillaries in the following) can be adjusted so that the capillaries correspond to the requirements of the application. Via selection of the size distribution of the pigments it is achieved that size distributions of the capillaries are varied in wide ranges.
However, problems occur in the transfer printing of the toner image onto the printing substrate in the transfer printing zone. The transfer of the toner image to the printing substrate is affected by the capillaries and by roughness in the printing substrate surface (paper, for example). Namely, the capillaries in the printing substrate suck carrier fluid from its surface into the printing substrate, with the result that the adhesion of the toner images to the printing substrate is reduced. For the transfer of the toner images onto the printing substrate it is advantageous if optimally all carrier fluid is kept on the surface of the printing substrate. For this the carrier fluid optimally may not be drawn into the capillaries in the printing substrate. The carrier fluid is required on the surface, namely for the electrophoresis process.
One method to reduce this problem is the transfuse method corresponding to U.S. Pat. No. 5,555,185. The carrier fluid is thereby no longer used as a transport medium in the transfer-printing of the toner images to the printing substrate. Instead of this, the toner particles are softened in the transfer unit via the application of high temperatures and then adapted with pressure to the printing substrate. A large, common surface thereby arises between printing substrate and toner image with large adhesion forces. The toner particles are transferred from the transfer roller onto the printing substrate in the transfer unit via these surface forces, although the carrier fluid is not used as a transport medium.